Technical Abstract:
Bioengineering of lignin to contain atypical components derived from other metabolic pathways is increasingly being pursued to custom design lignified cell walls that are more readily pretreated and saccharified for biofuel production or easily digested by livestock. Because plants produce such a diverse array of phenolics that could serve as alternate monomers for lignin formation, cell wall model studies are invaluable as a screening tool for identifying the most promising genetic engineering targets for biomass and forage crops. Our studies with such models demonstrated that copolymerization of normal monolignols with alternate monomers such as coniferyl ferulate, rosmarinic acid, epigallocatechin gallate, and several other catechin and gallate esters substantially improved the enzymatic saccharification of cell walls following mild chemical pretreatment. Incorporation of epigallocatechin gallate into lignin also yielded cell walls that were intrinsically more fermentable by rumen microflora than conventionally lignified cell walls. Most of these monomer conjugates improved cell wall utilization by incorporating readily cleavable ester linkages into the backbone of the lignin polymer. Ortho-OH groups on some alternate monomers also enhanced cell wall utilization by blocking the formation of benzyl ether cross-links between lignin and structural carbohydrates. By contrast, alternate monomers that incorporated amide or acetal functionalities into the lignin polymer usually failed to improve cell wall utilization. Most alternate monomers with hydrophilic moieties also failed to copolymerize with normal monolignols to form wall-bound lignin polymers. Overall, less than 25% of the phenolics tested in our studies proved compatible with cell wall lignification or useful for modulating the adverse effects of lignin on cell wall utilization. These findings highlight the value of testing alternate monolignols in biomimetic cell wall models prior to attempting their bioengineering into lignin biosynthesis.